In the manufacture of highly integrated semiconductor chips on wafers, the ever-increasing miniaturization of the structures on the semiconductor chip is responsible in particular for imposing ever greater requirements on the production installations and manufacturing processes used for the manufacture of the semiconductor chips. The stability and reproducibility both of the production installations and of the manufacturing processes decisively influence the yield and productivity during semiconductor chip production. Even small deviations from a prescribed form of behavior of a wafer production installation during production can lead to considerable worsening of the yield (i.e. a considerable increase in the defect rate of the semiconductor chips manufactured).
In general, the semiconductor chips are manufactured several at a time on wafers. Furthermore, in the manufacturing process a number of identical wafers are grouped into a logistical unit, a lot, and subjected together to semiconductor process steps.
For completely and exactly determining the product quality of the wafers of a lot or of the process for manufacturing these wafers, all the semiconductor chips would have to be subjected to test measurements after they have been completed to determine their properties and quality. However, this would require much too great an expenditure in terms of time and cost. Therefore, the Statistical Process Control (SPC) method is used for determining the quality of the manufactured semiconductor chips according to the prior art, arranged on the wafers. In the SPC method, a random sample of wafers is statistically selected from the lot of wafers and then test measurements are performed on them for determining the quality of the wafers. On the basis of the results of this test measurement, the quality of the wafers of the entire lot is concluded. It is assumed that the quality of all the wafers of the lot will then fluctuate about the measured quality values. The quality values determined in this way are used both for the determination of the cp value, which is a statement of the range of a distribution of the measured values, or in other words a measure of the smallest possible proportion of defective units (wafers) in the process that is expected when the position of the distribution is centred, and for the determination of the cpk value, which is a value which indicates how centrally the distribution of the measured values lies in relation to a prescribed specification, or in other words a measure of the expected proportion of defective units in the process.
A major aspect in the manufacture of semiconductor chips is also to detect possible deviations from a prescribed form of behavior in a chip production installation or during a manufacturing process at an early time and to take corresponding countermeasures. Consequently, the analysis and monitoring of machines, in particular of the chip production installations, and of the manufacturing processes takes on very great commercial significance. Furthermore, the analysis and monitoring of many process steps of the manufacturing process is of considerable significance, since it is usually only rarely possible to repair an intermediate product after a process step has been carried out. A functional test of a manufactured semiconductor chip is generally not provided within the SPC method until at the end of the manufacturing process, which leads to feedback of the results obtained into the manufacturing process only being possible very late. Measurements at the end of the manufacturing process also lead to unspecific results in the sense that possibly poor production quality of the wafer cannot necessarily be attributed to a specific processing step.
It is further known to perform inline measurements of interim process results, inline SPC measurements, for example of the layer thicknesses, the layer resistance or of line widths, etc., by means of the SPC method. This leads to additional measuring steps in the overall manufacturing process and is consequently time-consuming and costly, but increases the extent to which possible deficiencies in quality can be assigned to a specific processing step.
Furthermore, a method for improving the manufacturing process by means of continuous adaptations to the needs and established deficiencies of the wafers manufactured, the run-to-run method, is known. When the run-to-run method is used, the manufacturing process is constantly and continually (“from run to run”) controlled on the basis of measured process results (i.e. measurements on the products or intermediate products manufactured). Process parameters of a processing step are controlled on the basis of measured product results at short time intervals. The controlling by means of the run-to-run method is carried out by means of so-called run-to-run controllers.
However, to carry out the run-to-run method, measurements on wafers are necessary, in order to determine their product quality and control the process parameters in response to that. Since, as already mentioned, measurements for determining the product quality are both time-intensive and cost-intensive, the inline SPC measurements on randomly selected wafers are used as input variables for the control by means of the run-to-run method. These inline SPC measurements are used to provide the required indications of the product qualities, on the basis of which the run-to-run method can control the further production process.
GB 2,347,522 discloses a method and apparatus for the process control of semiconductor fabrication, in which a lot-based management computer performs management of wafers with a lot as a unit by managing a process condition for each lot.
U.S. Pat. No. 6,263,255 discloses an Advanced Process Control (APC) Framework, which automatically carries out process control operations through the design and development of a software framework that integrates factory, process and equipment control system.